A new laser-driven ion acceleration mechanism has been
identified using particle-in-cell simulations.
After a brief period of target normal sheath acceleration (TNSA)
[S. P. Hatchett, et al., Phys. Plasmas, 7, 2076 (2000)],
two distinct stages follow: first, a period of enhanced TNSA
during which the cold electron background converts entirely to
hot electrons, and second, the ``laser break-out afterburner'' (BOA)
when the laser penetrates to the rear of the target and generates a
large longitudinal electric field localized at the rear of the target
with the location of the peak field co-moving with the ions.
This mechanism allows ion acceleration to GeV energies at vastly
reduced laser intensities compared with earlier acceleration schemes.
The new mechanism enables the acceleration of carbon ions to greater
than 2 GeV energy at a laser intensity of only $10^{21}$~W/cm$^2$,
an intensity that has been realized in existing laser systems.
Other techniques for achieving these energies in the literature
[D. Habs et al., Progress in Particle and Nuclear Physics, 46,
375 (2001);
T. Esirkepov et al., Phys. Rev. Lett. 92, 175003-1 (2004)]
rely upon intensities of $10^{24}$~W/cm$^2$ or above, i.e.,
2-3 orders of magnitude higher than any laser intensity that has
been demonstrated to date. Also, the BOA mechanism attains higher
energy and efficiency than TNSA where the scaling laws
[Hegelich et al., Phys. Plasmas, 12, 056314 (2005)] predict carbon
energies of 50 MeV/u for identical laser conditions. In the early
stages of the BOA, the carbon ions accelerate as a
quasi-monoenergetic
bunch with median energy higher than that realized recently
experimentally [Hegelich et al., Nature, 441, 439 (2006)]. [Preview Abstract]

Recent breakthrough results reported in Nature\footnote{C.G.R.
Geddes et al., Nature {\bf 431}, 538 (2004); S.P.D. Mangles et
al., ibid., p. 535; J. Faure et al., ibid., p. 541.} demonstrate
that laser-plasma accelerators can produce high quality (e.g.,
narrow energy spread) electron bunches at the 100 MeV level that
may be useful for numerous applications. More recently, high
quality electron beams at 1 GeV were produced in experiments at
LBNL using 40 TW laser pulse interacting with a 3.3 cm plasma
channel\footnote{W.P. Leemans et al., submitted.}. In these
experiments, the accelerated electrons were self-trapped from the
background plasma, often attributed to the process of
wavebreaking. Using a warm fluid model, a general analytic
theory of wavebreaking has been developed that is valid for all
regimes of interest, i.e., arbitrary temperature and phase
velocity\footnote{C.B. Schroeder et al., Phys. Rev. E {\ bf
72}, 055401 (2005).}. This theory indicates that the maximum
electric field obtainable by a relativistic plasma wave is lower
that previously calculated. The relation between wavebreaking
and particle trapping is discussed, and various quantities, such
as the fraction of electrons trapped (i.e., the dark current),
are calculated\footnote{C.B. Schroeder et al., Phys. Plasmas
{\bf 13}, 033103 (2006).}. A variety of methods for particle
trapping relevant to present experiments, including 2D
wavebreaking, density ramps, and laser injection, will be
described\footnote{G. Fubiani et al., Phys. Rev. E {\bf 73},
026402 (2006).}. Limitations from dephasing and pump depletion
will be summarized. Also presented will be 2D and 3D simulations
modeling the production high quality electron beams from
laser-plasma accelerators. [Preview Abstract]

A great deal of interest in laser wakefield accelerators has been
generated since the discovery that they can produce high quality
(low emittance and low energy spread) ultra-short (less than 25
fs) relativistic electron beams.
This talk will cover the ongoing research led by Imperial College
at the Rutherford Appleton Laboratory and Lund Laser Centre.
By controlling the laser parameters including contrast ratio,
pulse duration and focusing geometry we can significantly improve
the quality and stability of the electron beam produced in
self-injected laser wakefield experiments.
We will also discuss the scaling of laser wakefield accelerators
to PW class lasers such as the Astra Gemini system at the
Rutherford Appleton Laboratory. [Preview Abstract]

Laser driven wakefield accelerators produce accelerating fields
thousands of times those achievable in conventional
radiofrequency accelerators, and recent experiments have produced
high energy electron bunches with low emittance and energy
spread. Challenges now include control and reproducibility of the
electron beam, further improvements in energy spread, and scaling
to higher energies. We present large-scale particle in cell
simulations together with recent experiments towards these goals.
In LBNL experiments the relativistically intense drive pulse
was guided over more than 10 diffraction ranges by plasma channels.
Guiding beyond the diffraction range improved efficiency by
allowing use of a smaller laser spot size (and hence higher
intensities) over long propagation distances. At a drive pulse
power of 9 TW, electrons were trapped from the plasma and beams
of percent energy spread containing $>$ 200pC charge above 80 MeV
with normalized emittance estimated at $ [Preview Abstract]

RF photocathodes are difficult to model but continue to be at the
forefront
of solutions to many applications, especially as high power FEL
sources.
Modeling the photoemission process requires a high degree of
computational
mesh resolution to resolve geometrical and surface finish
features, or
simply fine spatial scale phenomena. The new Finite-Element (FE)
MICHELLE
[1] two-dimensional (2D) and three-dimensional (3D) steady-state and
time-domain particle-in-cell (PIC) code has been employed
successfully by
industry, national laboratories, and academia and has been used
to design
and analyze a wide variety of beam sources and devices. In
particular, the
MICHELLE code has the ability to resolve small spatial scales,
and is a good
choice for photoemission modeling. To investigate the application
of the
Electrostatic time-domain model to emission properties of
photocathodes, two
code models are needed; an EM frequency-domain code and a PIC
code. We use
the STAR ANALYST [2] code for the Frequency Domain solutions and the
NRL/SAIC MICHELLE code for the PIC solutions. The RF fields from
ANALYST are
imported into the MICHELLE code and clocked in time. MICHELLE
adds the self
fields and emits the beam according to an emission rule. For the
photoemission process, we employ the NRL photoemission model [3],
and can
capture detailed spatial and temporal effects of the emission
surface finish
and beam development. In the talk, we will consider an example that
investigates the effects of fine scale surface imperfections on the
photoemission process.
\newline
\newline
[1] John Petillo, et al., ``The MICHELLE
Three-Dimensional Electron and Collector Modeling Tool: Theory
and Design,'' IEEE Trans. Plasma Sci., vol. 30, no. 3, June 2002,
pp. 1238-1264.
\newline
[2] Analyst is a commercial finite-element package for
electromagnetic
design. www.staarinc.com.
\newline
[3]K. Jensen, et al., ``The Quantum Efficiency of Dispenser
Photocathode:
Comparison of Theory to Experiment'' Applied Physics Lett. 85,
22, 5448,
2004.
[Preview Abstract]